1. Field of the Invention
The present invention relates to a capacitance-type external-force detecting device typically used for detecting a physical parameter such as an angular velocity or acceleration based on a variation in capacitance.
2. Description of the Related Art
Normally, capacitance-type external-force detecting devices have been used, for example, as angular velocity sensors or acceleration sensors. By causing a moveable unit provided on a substrate to be displaced in accordance with an external force, such as an angular velocity or acceleration, which causes the capacitance between the substrate and the moveable unit to be varied, these sensors detect the variation as an indication of the magnitude of the external force.
Two examples of an external-force detecting device are disclosed in Japanese Unexamined Patent Application Publication No. 11-337342 and Japanese Unexamined Patent Application Publication No. 9-318656. These devices include a substrate and a moveable unit which is supported by the substrate via a supporting beam (and which is moveable in the detecting direction). A comb-shaped fixed electrode is located on the substrate and has a plurality of fixed finger electrodes which are spaced apart in the detecting direction. A comb-shaped moveable electrode is provided on the moveable unit and has a plurality of moveable finger electrodes which are provided between the fixed finger electrodes of the fixed comb electrode so that the moveable finger electrodes mesh with the fixed finger electrodes. A capacitance detecting circuit detects the displacement of the moveable finger electrodes relative to the fixed finger electrodes when an external force causes the moveable unit to be displaced in the detecting direction as a function of the variation in capacitance between the fixed finger electrodes and the moveable finger electrodes.
The comb-shaped fixed and moveable electrodes are formed by applying a fining process, such as etching processing, to, for example, silicon materials. The fixed finger electrodes are provided so as to line up at a predetermined spacing in the detecting direction of the external force. The moveable finger electrodes are provided between the fixed finger electrodes so as to mesh with the fixed finger electrodes with gaps therebetween.
A respective pair of fixed finger electrodes are disposed on opposite sides of most of the moveable finger electrodes in the detecting direction. The moveable finger electrode and one of the pair of fixed finger electrodes constitute a first capacitor therebetween and the moveable finger electrode and the other one of the fixed finger electrodes constitute a second capacitor therebetween. These two capacitors are connected in parallel to the capacitance detecting circuit via the fixed and the moveable finger electrodes.
The moveable finger electrode is closer to one of the fixed finger electrodes than it is to the other of the fixed finger electrodes, which makes the inter-electrode distance of the first capacitor shorter than that of the second capacitor.
When an external force such as the angular velocity or the acceleration is applied to the substrate, an inertia force and the like cause the moveable unit to be displaced in the detecting direction, which displaces the moveable finger electrode with respect to the fixed finger electrodes. Since this (the inter-electrode distance) varies the capacitance of the capacitor formed between these finger electrodes, the capacitance detecting circuit can detect the variation in capacitance between the fixed and moveable finger electrodes as an indication of the magnitude of the angular velocity, the acceleration, or the like.
For example, when the moveable finger electrode is displaced in one of the detecting directions, the inter-electrode distance of the first capacitor is decreased, increasing the capacitance thereof, and the inter-electrode distance of the second capacitor is increased, decreasing the capacitance thereof. Since the variation in overall capacitance of the capacitors is counterbalanced, and is decreased, detecting the variation in the capacitances becomes difficult.
Because the inter-electrode distance of the first capacitor is shorter than that of the second capacitor, minute variations in the inter-electrode distances cause the variation in the capacitance (the variation ratio of the capacitance) of the first capacitor to be greater than that of the second capacitor.
When the external force displaces the moveable finger electrode, since the capacitances of the first and second capacitors vary at different rates, the overall (net) capacitance of these capacitors varies. Therefore, the external-force detecting sensitivity of the capacitance detecting circuit can be improved.
In the foregoing art, there has been a demand for improving the external-force detecting sensitivity as much as possible by causing the overall (net) capacitance of the capacitor consisting of the first and second capacitors to vary greatly in accordance with displacement of the moveable finger electrode. One method for increasing the difference between the variation rates of the capacitances of these capacitors is to form the inter-electrode distance of the first capacitor to be shorter or that of the second capacitor to be longer.
However, it is difficult to finely adjust the inter-electrode distance of the first capacitor, since the processing accuracy and the like of etching processing restricts the minimum dimension between the finger electrodes. As a result, there is a limit to the improvement in the detecting sensitivity using a fining process. When the inter-electrode distance of the second capacitor is formed to be long, the dimensions of the entire substrate including the fixed electrode unit and moveable finger electrode must be large.
Therefore, there are problems in that the related art finds it difficult to appropriately set the inter-electrode distances between the fixed and moveable finger electrodes and finds it impossible to facilitate improvement in the detecting sensitivity while the substrate and the like are miniaturized.
The present invention is made in view of the problems of the foregoing related art. An object of the present invention is to increase the sensitivity of the capacitance-type external-force detecting device without unduly increasing the size of the device.
To this end, according to a first aspect of the present invention, there is provided an external-force detecting device comprising:
n fixed electrodes supported by said substrate, n being an integer greater than 1;
n moveable electrodes which are located at nominal positions relative to the fixed electrodes when they are at rest and are moveable in a detection direction in response to an external force applied thereto so as to vary the relative locations of the fixed and moveable electrodes;
n/2 electrode pairs, each electrode pair including a respective fixed electrode and a respective moveable electrode that together define a variable capacitance whose capacitance varies as a function of the distance therebetween, the electrode pairs being divided into first and second sets of electrode pairs, the fixed and moveable electrodes of the first set of electrode pairs being spaced from one another by a first inter-electrode distance when the moveable electrodes are in their stationary positions, the fixed and moveable electrodes of a second set of electrode pairs being spaced from one another by a second inter-electrode distance when the moveable electrodes are in their stationary positions, the dimension ratio of the first inter-electrode distance to the second inter-electrode distance falling within the range of 1:2 to 1:5; and
a detector for detecting the displacement of the moveable electrodes relative to the fixed electrodes as a function of the variation in the overall capacitances of the first set of electrode pairs to the capacitances of the second set of electrode pairs resulting from movement of the moveable electrodes in response to an application of the external force.
When the capacitance-detecting unit outputs a voltage signal in accordance with the variation in the ratio of the overall capacitance of the first and second electrode pairs, the sensitivity per unit length can be maximized by setting the first and second inter-electrode distance to be within the range of 1:2 to 1:5. Hence, when, for example, the capacitance-detecting unit is constructed using a high input-impedance field-effect transistor, a high input-impedance operational amplifier, or the like, the dimension ratio of the first to the second inter-electrode distances, and the detecting sensitivity for the unit length in the detecting direction can be increased. As a result, even when the moveable electrodes are is displaced slightly in the detecting direction, since the voltage signal variation is large and the detecting sensitivity is increased. This makes it possible to decrease electrode distances and the sensor can be miniaturized while simultaneously improving its detecting sensitivity.
In the preferred embodiment, at least one of the moveable electrodes forms two separate electrode pairs with two different fixed electrodes and at least some of the fixed electrodes form two separate electrode pairs with two different moveable electrodes.
The fixed electrodes are preferably provided on the substrate and the movable electrodes are preferably supported by a movable platform which is supported on the substrate by at least one support beam.
In the preferred embodiment, the fixed electrodes are part of one or more fixed comb electrodes and the moveable electrodes are part of one or more comb electrodes. Finger electrodes of the fixed and moveable comb electrodes are interdigitated so as to form the various electrode pairs.
The capacitance-detecting unit may include a field-effect transistor for converting the variation in the ratio of the overall capacitances of the first and second sets of electrode pairs into a voltage signal and for outputting the converted voltage signal.
The detecting sensitivity per unit length in the detecting direction can be increased by forming ratio of the first and second inter-electrode distances to the second inter-electrode distance to be within a range of 1:2 to 1:5.
The capacitance-detecting unit may include, for example, a field effect transistor or an operational amplifier for converting the variation in the ratio of the overall capacitances between the first and second electrode pairs into a voltage signal and for outputting the converted voltage signal.
According to a second aspect of the present invention, a capacitance-type external-force detecting device includes an external force detecting device, comprising:
n fixed electrodes supported by said substrate, n being an integer greater than 1;
n moveable electrodes, the moveable electrodes being located at nominal positions relative to the fixed electrodes when they are at rest and being moveable in a detection direction in response to an external force applied thereto so as to vary the relative locations of the fixed and moveable electrodes;
n/2 electrode pairs, each electrode pair including a respective fixed electrode and a respective moveable electrode that together define a variable capacitance, whose capacitance varies as a function of the distance therefrom, the electrode pairs being divided into first and second sets of electrode pairs, the fixed and moveable electrodes of one half of the electrode pairs being spaced from one another by a first inter-electrode distance when the moveable electrodes are in their nominal positions, the fixed and moveable electrodes of a second set of electrode pairs being spaced from one another by a second inter-electrode distance when the moveable electrodes are in their nominal positions, the ratio of the first inter-electrode distance to the second inter-electrode distance falling within the range of 1:1.7 to 1:3.5; and
a detector for detecting the displacement of the moveable electrodes relative to the fixed electrodes as a function of the variation in the magnitude of the overall capacitances between the electrode pairs.
When the capacitance-detecting unit outputs the voltage signal in accordance with the variation in the overall magnitude of capacitances between the electrode pairs, the dimension ratio of the first and second inter-electrode distances is constructed to be within a range of 1:1.7 to 1:3.5. Hence, when, for example, the capacitance-detecting unit is constructed to output the voltage signal substantially in proportion to the amount of variation in capacitance, the dimension ratio of the first inter-electrode distance to the second inter-electrode distance can be appropriately set between the fixed electrode unit and the moveable-side electrode unit, and the detecting sensitivity for the unit length in the detecting direction can be increased. As a result, since the inter-electrode distances and the like can be formed to be short while this detecting sensitivity for the external force can be maintained in a preferable manner, miniaturization of a device and the improvement in its detecting sensitivity can be compatible.
The comb-shaped fixed electrode units and the comb-shaped moveable-side electrode units may include a single crystal or a polycrystal of silicon material.
Since comb-shaped fixed and moveable-side electrodes can be constructed using a single crystal or polycrystal silicon material, for example, mere application of a fining process such as an etching process to the silicon material can simultaneously and effectively form these comb-shaped electrodes.